Utilizing the marine mollusc Aplysia as a simple test system, I will quantitatively analyze the neural organization underlying three elementary types of behavior: 1) a simple graded reflex, defensive gill-withdrawal, 2) two all-or-none fixed acts, inking behavior and opaline secretion, and 3) a simple fixed action pattern, spontaneous respiratory pumping. For each of these behaviors there is presently a good qualitative understanding. This proposal is an attempt to extend these previous studies to a more quantitative level by determining to what extent a quantitative knowledge of individual neural elements can account for each behavior. To accomplish these ends, I will use the combined techniques of systems analysis, cellular neurophysiology, voltage clamp analysis and computer simulation. For the gill-withdrawal reflex I will analyze the relationship between the tactile simulus to the skin and the magnitude and time course of the reflex mediated gill contractions by obtaining transfer functions of the input-output characteristics of the individual sensory and motor elements. For inking behavior I will identify pre-motor neurons and incorporate their synaptic actions into a previously developed Hodgkin-Huxley formulation of the ink motor neurons. For opaline secretion I will compare whether the membrane-biophysical and synaptic connectivity properties of neurons mediating this behavior are similar to those responsible for the features of inking behavior. For spontaneous respiratory pumping I will further examine the underlying neural circuit and quantitatively analyze the mechanisms underlying the generation of the spontaneous burst which initiates the behavior. Where possible I will develop mathematical formulations which will be simulated on a digital computer to determine how well the neural processes account for the behaviors. These analyses and simulations may not only lead to a greater understanding of these three basic behavioral categories, but may also lead to a refinement of techniques which may then be more readily applied to the analysis of behavior and behavioral modifications in more complex organisms.